Crop the image that you want to use to the exact size of the dashboard (not the container you just resized). Switch back to the dashboard pane and drag in an image object. Choose your file. Drag the image to the back (=top) in the item hierarchy of the dashboard. Select the floating main container and add a white background to it.

Backgrounds appear very similar to shading but operate completely differently. To add a background to a dashboard object, select the object and open the Layout pane. About halfway down is the Background menu. Any type of dashboard object can have a background including Containers, Blank objects, and even Web Pages.